Rejection Sampling - ChemLactica

Overview

Rejection sampling is an advanced fine-tuning strategy that iteratively generates and filters molecular candidates based on desired properties. This approach is particularly effective for molecular optimization tasks where you want to improve specific properties while maintaining structural similarity.

How It Works

Generate Candidates

The model generates multiple molecular candidates based on a lead molecule and similar structures

Filter by Criteria

Candidates are filtered based on:

QED (drug-likeness) score
Tanimoto similarity to lead molecule
Other custom properties

Create Training Samples

Accepted molecules are formatted into training samples with their properties

Fine-tune Model

The model is fine-tuned on the filtered samples

Iterate

Process repeats for multiple rounds, progressively improving the model

Quick Start

python chemlactica/rejection_sampling_ft.py \
  --from_pretrained facebook/galactica-125m \
  --model_config 125m \
  --valid_data_dir ./data/valid \
  --rounds 10 \
  --steps_per_round 100 \
  --train_batch_size 8 \
  --max_learning_rate 1.0e-4 \
  --eval_steps 50 \
  --save_steps 100 \
  --checkpoints_root_dir ./checkpoints/rejection_sampling \
  --experiment_name mol_optimization \
  --device cuda:0 \
  --seed 42

Command-Line Arguments

Required Arguments

string

required

Path to pretrained model or model identifier

string

required

Model configuration name (e.g., 125m, 1.3b)

string

required

Directory containing validation data in JSONL format

integer

required

Number of rejection sampling rounds to perform

integer

required

Number of training steps per round (also number of samples generated)

integer

required

Training batch size

float

required

Maximum learning rate for training

integer

required

Steps between evaluation runs

integer

required

Steps between checkpoint saves

string

required

Root directory for saving checkpoints

string

required

Device to use for generation (e.g., cuda:0, cpu)

Optional Arguments

string

default:"none"

Name for the experiment

integer

Validation batch size (defaults to training batch size)

integer

default:"0"

Number of dataloader workers

integer

default:"1"

Gradient accumulation steps

boolean

default:"false"

Enable gradient checkpointing

boolean

default:"false"

Use Flash Attention

boolean

default:"true"

Enable Aim tracking

string

Directory for tracking data

integer

default:"42"

Random seed for reproducibility

Generation Configuration

The rejection sampling process uses two sets of generation parameters defined in rejection_sampling_configs.py:

Similar Molecule Generation

sample_gen_args = {
    "max_new_tokens": 50,
    "temperature": 1.0,
    "repetition_penalty": 1.0,
    "do_sample": True,
    "eos_token_id": 2
}

integer

default:"50"

Maximum tokens to generate for similar molecules

float

default:"1.0"

Sampling temperature (higher = more diverse)

float

default:"1.0"

Penalty for repeating tokens

Target Molecule Generation

rej_sample_args = {
    "max_new_tokens": 300,
    "temperature": 1.0,
    "repetition_penalty": 1.0,
    "do_sample": True,
    "num_return_sequences": 20,
    "eos_token_id": 20,
}

integer

default:"20"

Number of candidate molecules to generate per prompt

integer

default:"300"

Maximum tokens for target molecule generation

Molecular Scoring

Molecules are evaluated using multiple metrics:

QED Score

# From rejection_sampling_utils.py:55
def compute_qed(smiles: str):
    return qed(Chem.MolFromSmiles(smiles))

Quantitative Estimation of Drug-likeness (0-1 scale, higher is better).

Tanimoto Similarity

# From rejection_sampling_utils.py:48
def tanimoto_dist_func(
    smiles1: str, 
    smiles2: str, 
    fingerprint: FingerprintType=FingerprintType.Morgan
):
    return DataStructs.TanimotoSimilarity(
        get_morgan_fingerprint(smiles1),
        get_morgan_fingerprint(smiles2)
    )

Measures structural similarity between molecules (0-1 scale).

Combined Score

# From rejection_sampling_utils.py:74
def score(self):
    return self.morgan_sim_to_lead + self.qed

Molecules are ranked by the sum of similarity and QED scores.

Optimization Criteria

The default optimization targets molecules with:

# From rejection_sampling_ft.py:266-269
optimized_molecule_mask = np.bitwise_and(
    df_samples["qed"].values >= 0.9,
    df_samples["morgan_sim_to_lead"].values >= 0.4,
)

QED ≥ 0.9: High drug-likeness
Tanimoto similarity ≥ 0.4: Maintains structural features of lead

Prompt Format

The rejection sampling generates prompts in this format:

</s>[SIMILAR]{mol1_smiles} {similarity}[/SIMILAR][SIMILAR]{mol2_smiles} {similarity}[/SIMILAR][QED]{target_qed}[/QED][START_SMILES]{target_smiles}[END_SMILES]</s>

Example

</s>[SIMILAR]CC(C)Cc1ccc(C)cc1 0.92[/SIMILAR][SIMILAR]c1ccc2[nH]ccc2c1 0.88[/SIMILAR][QED]0.95[/QED][START_SMILES]CC(C)Cc1ccc2[nH]ccc2c1[END_SMILES]</s>

Training Process

Round-by-Round Workflow

For each round (from rejection_sampling_ft.py:235-299):

Load Generator Model

if trainer.state.global_step == 0:
    generator_checkpoint_path = from_pretrained
else:
    generator_checkpoint_path = os.path.join(
        training_args.output_dir, 
        f"checkpoint-{trainer.state.global_step}"
    )

Generate Samples

train_ds_name = generate_dataset(
    checkpoint_path=generator_checkpoint_path,
    run_hash=experiment_hash,
    round=i,
    num_samples=steps_per_round,
    max_similars_in_prompt=5,
    lead_molecule=lead_molecule,
    use_flash_attn=use_flash_attn,
    device=device,
    seed=seed,
)

Filter and Score

df_samples = pd.read_csv(train_ds_name)
optimized_molecule_mask = np.bitwise_and(
    df_samples["qed"].values >= 0.9,
    df_samples["morgan_sim_to_lead"].values >= 0.4,
)

Train on Samples

rej_sampled_samples = list(df_samples["samples"].values)
random.shuffle(rej_sampled_samples)
for batch_of_texts in batches:
    trainer.step(texts=batch_of_texts)

Save Checkpoint

trainer._save_checkpoint(model=None, trial=None)

Learning Rate Schedule

Rejection sampling uses polynomial decay with warmup:

# From rejection_sampling_ft.py:157-163
lr_scheduler = get_polynomial_decay_schedule_with_warmup(
    optimizer,
    num_warmup_steps=train_config["warmup_steps"],
    num_training_steps=steps_per_round * rounds,
    lr_end=0.1 * train_config["max_learning_rate"],
    power=1.0,
)

This creates a linear decay from max LR to 10% of max LR over all rounds.

Example: Lead Optimization

Define Lead Molecule

# From rejection_sampling_ft.py:245-247
lead_molecule = "c1ccc(-c2cc(N3C[C@H]4[C@@H]5CC[C@@H](O5)[C@H]4C3)c3ccccc3[nH+]2)cc1"

Run Optimization

python chemlactica/rejection_sampling_ft.py \
  --from_pretrained ./checkpoints/pretrained/galactica-125m \
  --model_config 125m \
  --valid_data_dir ./data/valid \
  --rounds 20 \
  --steps_per_round 200 \
  --train_batch_size 8 \
  --max_learning_rate 5.0e-5 \
  --eval_steps 100 \
  --save_steps 200 \
  --checkpoints_root_dir ./checkpoints/rejection_sampling \
  --experiment_name lead_optimization \
  --dataloader_num_workers 4 \
  --gradient_accumulation_steps 2 \
  --device cuda:0 \
  --flash_attn \
  --track \
  --track_dir ./aim_logs \
  --seed 42

Monitor Progress

The training logs will show:

---------Rej Sampling ROUND 1---------
Generator model: ./checkpoints/pretrained/galactica-125m
Found optimized molecule: True

Successful rounds find molecules meeting the optimization criteria (QED ≥ 0.9, similarity ≥ 0.4).

Advanced Configuration

Custom Similarity Thresholds

Modify the prompt generation in generate_dataset:

# Adjust similarity range
morgan_sim_to_lead=random.uniform(0.85, 0.95)  # Stricter similarity

Custom QED Targets

# Target higher QED values
input_text += f"[QED]{random.uniform(0.95, 0.99):.2f}[/QED]"

Multiple Lead Molecules

Modify the training loop to alternate between different leads:

lead_molecules = ["SMILES1", "SMILES2", "SMILES3"]
for i, lead in enumerate(cycle(lead_molecules)):
    train_ds_name = generate_dataset(
        lead_molecule=lead,
        # ... other args
    )

Data Storage

Generated samples are saved as CSV files:

# From rejection_sampling_utils.py:110-113
base_path = f"/nfs/dgx/raid/chem/data/rej_sampling_data/{run_hash}"
ds_file_name = f'{base_path}/round:{round}_hash:{model_name[0][:6]}_step:{model_name[1].split("-")[-1]}_date:{formatted_date_time}.csv'

Each CSV contains:

samples: Full formatted training sample
smiles: Generated SMILES string
qed: QED score
morgan_sim_to_lead: Tanimoto similarity (Morgan fingerprint)

Performance Tips

Generation Speed

Use --flash_attn for faster generation
Adjust num_return_sequences to balance diversity and speed
Use GPU for generation (--device cuda:0)
Consider batch generation for multiple leads

Sample Quality

Increase max_similars_in_prompt (default 5) for more context
Adjust temperature for more/less diversity
Filter invalid SMILES early to save computation
Use canonical and Kekulé SMILES for consistency

Memory Management

# From rejection_sampling_ft.py:261-263
gc.collect()
torch.cuda.empty_cache()

Clear cache between rounds to prevent OOM

Troubleshooting

No Optimized Molecules Found

Relax optimization criteria (lower QED/similarity thresholds)
Increase num_return_sequences for more candidates
Try different lead molecules
Increase temperature for more diversity

Invalid SMILES Generated

Check tokenizer is properly configured
Verify model was trained on SMILES data
Ensure proper formatting of start/end tokens
Consider adding SMILES validation callback

Training Instability

Reduce learning rate
Increase warmup steps
Use gradient clipping (controlled by global_gradient_norm)
Monitor gradient norms in Aim

Next Steps

Pretraining

Learn about pretraining models from scratch

Configuration

Explore all training configuration options

​Overview

​How It Works

​Quick Start

​Command-Line Arguments

​Required Arguments

​Optional Arguments

​Generation Configuration

​Similar Molecule Generation

​Target Molecule Generation

​Molecular Scoring

​QED Score

​Tanimoto Similarity

​Combined Score

​Optimization Criteria

​Prompt Format

​Example

​Training Process

​Round-by-Round Workflow

​Learning Rate Schedule

​Example: Lead Optimization

​Define Lead Molecule

​Run Optimization

​Monitor Progress

​Advanced Configuration

​Custom Similarity Thresholds

​Custom QED Targets

​Multiple Lead Molecules

​Data Storage

​Performance Tips

​Troubleshooting

​No Optimized Molecules Found

​Invalid SMILES Generated

​Training Instability

​Next Steps

Pretraining

Configuration

Overview

How It Works

Quick Start

Command-Line Arguments

Required Arguments

Optional Arguments

Generation Configuration

Similar Molecule Generation

Target Molecule Generation

Molecular Scoring

QED Score

Tanimoto Similarity

Combined Score

Optimization Criteria

Prompt Format

Example

Training Process

Round-by-Round Workflow

Learning Rate Schedule

Example: Lead Optimization

Define Lead Molecule

Run Optimization

Monitor Progress

Advanced Configuration

Custom Similarity Thresholds

Custom QED Targets

Multiple Lead Molecules

Data Storage

Performance Tips

Troubleshooting

No Optimized Molecules Found

Invalid SMILES Generated

Training Instability

Next Steps